Novel Controllable Commutated Converter (CLCC) and Rapid Defense Method for Commutation Failure

Commutation failure (CF) is an unique fault of traditional line commutated converter based high voltage direct current transmission system (LCC-HVDC). It is usually induced by AC system turbulence or fault, resulting in converter malfunction and swinging of power. For multi-infeed DC systems, CF may occur among several DC terminals, imposing significant safety threat on power grid. In order to solve CF completely, this paper proposed a novel converter topology, based on a combination of semi-controlled thyristors and fully-controlled IGBTs to aid the turning-off process of thyristors and realize forced commutation between bridge arms. Two operation modes were also designed and compared for the new converter. And a rapid detection strategy using the difference between arm current and its delay value is presented, which can identify the specific CF arm as fast as its commence, and triggering the mode switch, hence avoid CF at the early stage. Finally, a testing HVDC system model is developed from Cigre Benchmark model, adopting new CLCC converter at the inverter side. Using this model, AC grounding fault is simulated and the results shows that the proposed converter and its rapid defense strategy is efficient, and controllable commutation and relatively high power transmission is attained during fault, showing excellent fault ride-through and system support capacity.